Electrolytic recording sheets



March 23, 1965 E. D. HoRNE ETAL 3,174,856

ELECTROLYTIC RECORDING SHEETS March 23, 1965 E. D. HoRNE ETAL 3,174,856

ELEcTEoLYTEc RECORDING SHEETS Filed June 9, 1961 C5 Sheets-Sheet .'5

3,174,856 ELECTRLYTEC RECGRDING SHEETS Einar D. Home, Hudson, Wis., and Bavid A. Morgan, St.

Paul, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed lune 9, 1961, Ser. No. 116,092 11 Claims. (Cl. 96-1) This invention relates to new and useful sheets for the reproduction of visible images. In one aspect, this invention relates to improved image recording sheets which can be developed by electrolytic techniques. In another aspect, this invention relates to a method for the preparation of such sheets and to a method for their use in image reproduction.

Photoconductive coatings and sheet materials have been suggested for various purposes, including the reproduction of light images. In the recently developed Electrofax process an ordinary paper base is coated with photoconductive zinc oxide in a resinous binder. The sheet is electrostatically charged, exposed to a light image, and the remaining electrostatic charge developed with a charged powder as in the conventional xerographic process, except that the image is permanently retained on the coated paper, the powder being fused in place. The sensitive papers used in these processes have required that the sheet surface be capable or" retaining a high voltage electrostatic charge under dark conditions, and of permitting such charge to be dissipated when the surface is exposed to light.

A recently developed image reproduction process involves electrolytically developing permanent and visible images on suitable photoconductive copysheets after exposure to light images. This method, described more fully in United States application Serial Number 575,070, led March 30, 1956, now United States Patent No. 3,010,883, includes the electrolysis of an electrolytic developer and particularly the electrodeposition of a metallic or other visibly distinct coating at the exposed photosensitive surface, usually by electrolytic reduction. No preliminary charging of the copysheet is required, and the copy produced needs no further heating or other processing to render the image permanent. However, the successful application of the electrolytic method has been found to require, among other things, that the sensitive sheet be strongly photoconductive and have certain other conductivity requirements rather than merely the capacity to hold and dissipate an electrostatic charge. These sheets, as disclosed in greater detail in US. Serial Number 629,529, filed October 28, 1957, now United States Patent No. 3,010,884, generally comprise a photoconductive layer of zinc oxide particles in an insulative organic resinous binder superimposed on a contiguous electrically conductive layer, the combined layers having a conductivity of at least about i-7 mho/cm. on exposure to 1300 footcandles of incident light (i.e., light conductivity) and a dark conductivity not greater than about one-twentieth of the light conductivity. A preferred organic resinous binder is a copolymer of butadiene and styrene (e.g., 30 mol percent butadiene copolymer, Pliolite S-7, Goodyear Tire and Rubber Co.). Copysheets employing photoconductive indium oxide and their preparation are described in US. Serial Number 848,219, filed October 23, 1959.

Although the copysheets prepared in the above manner have a light sensitive surface which visually is reasonably white, the appearance of the sheet has generally been slightly off-white or gray, particularly in comparison to ordinary bond letterhead stationery. Attempts have been made to improve the whiteness of these photoconductive copysheets by incorporating small -amounts of inert whitnited States Patent ice l ening agents, such as titanium dioxide, etc., into the photoconductive layer. However, such use of white fillers seriously and adversely affects the sensitivity of the copysheet, significantly lowering both response rate and contrast at a given -set of exposure and development conditions.

lt is therefore an object of this invention to prepare an improved photoconductive copysheet suitable for use in image reproduction processes, particularly the electrolytic image reproduction process.

It is a further object of this invention to prepare an improved photoconductive copysheet of greater whiteness.

Still another object of this invention is to provide a photoconductive copysheet having both improved whiteness and greater sensitivity.

Yet another object of this invention is to provide a photoconductive copysheet with a greater light conductance to dark conductance ratio and with a higher photoresponse rate.

Further it is an object of this invention to provide an improved photoconductive copysheet capable of reproducing images with a higher contrast.

Other objects and advantages will be apparent from the following disclosure.

According to this invention, improved photoconductive copysheets contain 'an organic resinous binder having from about 72 mol percent to about 88 mol percent of a vinyl benzene in polymer form and, correspondingly, from about l2 mol percent to about 28 mol percent of a polymerized conjugated diene, such as butadiene, pentadiene, etc. This may be readily achieved by using a single copolymer of the conjugated diene and the vinyl benzene. In another embodiment such improved results may also be achieved by utilizing a binder comprising from about 0.1 to about 70 weight percent, prefer-ably from about 5 to about 65 weight percent, of a vinyl benzene homopolymer and from 30 to about 99.9 weight percent, preferably from about 35 to about 95 weight percent, of a copolymer of butadiene and a vinyl benzene, preferably having from about 20 to about 50` mol percent butadiene. Vinyl benzene, as used herein, includes both the unsubstituted vinyl benzene, i.e., styrene, and the substituted vinyl benzene, e.g., alkyl styrenes, alkoxystyrenes, halostyrenes, carboxystyrenes, aminostyrenes, hydroxystyrenes, nitrosytrenes, etc. Such substituted vinyl benzenes are shown in Vinyl and Related Polymers, by C. E. Schildknecht, chapter III, John Wiley & Sons, NY., 2. These binder compositions, when employed as matrices for photoconductive particulate materials, such as Zine oxide, cadmium sulfide, indium oxide, etc., increase the light conductance of the resultant copysheets when coated onto an electrically conductive backing, such as clean aluminum foil, as well as increase the difference between the light and dark conductance values. This permits a whitener or 'a filler such `as an insulating coloring agent or a white, insulating, non-photoconductive inorganic pigment, eg., titanium dioxide, zinc sulfate, barium sulfate, etc., to be added `to these coating compositions, preferably up to a maximum of about 30 weight percent of total pigment, i.e., combined whitener and photoconductive pigment. Since the difference between the light and dark conductance values (i.e., photoconductance) is a measure of sensitivity and also of the available contrast, the greater difference is highly desirable for image reproduction, even without added whitener. Usually when titanium dioxide is employed, between 10 and 20 weight percent of TiO2 based on total pigment is preferred. This provides a copysheet which not only has greatly improved whiteness but which `also has greater speed and contrast. For copysheets of this invention the ratio of total pigment to binder ranges from 2:1 to about 7:1, preferably from about 3.5 :l to about 6: 1. Higher total pigment to binder ratios may be used, although the flexibility of the copysheet and/or the adhesion of the photoconductive layer to the electrically conductive layer are not generally as acceptable for commercial copysheets. The pigment to binder ratio must be at least suticient to provide an electrically conductive path between the exposed surface of the photoconductive layer and the electrically conductive layer.

A further advantage realized by this invention is the ability to reduce the coating thickness of the photoconductive layer without adversely affecting the appearance or performance of the resultant copysheet, thereby effecting marked economies in material cost per unit. This is apparently due to the high opacifying power of the whitening agents, particularly titanium dioxide. As used herein,

the properties of whitener or whitening agent are relative to the particular photoconductive particles used, particularly zinc oxide.

Since the polymers employed in the binder composition of this invention are insulating materials, no theoretical explanation is known for the marked and unexpected improvements ach-ieved when the composition contains vinyl benzene monomeric units within the abovementioned range. Although any butadiene-vinyl ,benzene copolymer with at least about 10 mol percent of butadiene may be used in conjunction with a vinyl benzene homopolymer, those copolymers having between about 20 and about 50 mol percent butadiene are more readily available and are moreover preferred, particularly the copolymers with styrene and vinyl toluene. The homopolymer, when used, maybe a homopolymer of any of a number of vinyl benzenes, ranging in molecular weight `from the low molecular weight tetramer to the high molecular weight polymers (i.e., average molecular weights above about 10,000), for example Dow Styron 700, Koppers Type 2, Koppers Type 3, polyvinyltoluene, polystyrene tetramer, etc. For any combination of the above described homopolymers and copolymers, the weight ratio of these polymeric substituents is selected to provide an overall vinyl benzene content within the ranges set forth earlier, provided that the resultant binder mixture is essentially non-tacky and is tightly adherent to the electrically conductive material used in the copysheet construction. This can readily be determined by coating the binder mixture onto a test panel, e.g., aluminum, and testing the dry coating by conventional techniques both for tack and for adhesion. Brittleness is generally to be avoided in flexible copysheets and can also be tested by this simple technique. When a mixture of vinyl benzene homopolymer and vinyl benzenebutadiene copolymer is used, within the polymer weight ratios set forth earlier, it has been found, for example, that lesser amounts of the lower molecular weight styrene homopolymers, such as the tetramer, in the binder mixture provide better adhesion and have less tack than larger amounts of the same homopolymer, though the sensitivity is generally related to the relative weight ratio o-f the two different polymers. It is generally preferable, therefore, to utilize only the higher molecular weight polymers with average molecular weight above about 10,000. Both the sensitivity characteristics and the physical properties of the coating (e.g., adhesion to electrically conductive backing and tack) must therefore be taken into consideration in preparing commercial photoconductive copysheets with the advantages described herein.

In some instances the flexibility of the resultant photoconductive coatings may be enhanced by the incorporation of minor amounts of plasticizing agents, up to about 5 (preferably below l) weight percent of the total polymeric binder. Some suitable well-known plasticizers include tricresyl phosphate, dioctyl phthalate, etc. The lower molecular weight polystyrene polymers also provide a plasticizing effect. In referring to the binder compositions herein it is to be understood that the binders consist essentially of the aforementioned polymers with or without such minor amounts of a plasticizer to modify the physical properties of the coating.

When a vinyl benzene homopolymer is used, it is essential that the relative weight proportions of the vinyl benzene copolymer and the vinyl benzene homopolymer be maintained within the earlier described limits. If the resinous binder contains above about 70 weight percent of the homopolymer the dark conductance values increase sharply and the photoconductance is significantly reduced, as shown in FIGURE 5. Moreover, with the higher molecular weight homopolymers, such as Dow 700, such high homopolymer content tends to produce relatively brittle coatings. With less than about one weight percent of the vinyl benzene homopolymer the improvement in both speed and contrast is observed but is less than optimum. It is possible, however, to employ one or more of these homopolymers in conjunction with one or more of these copolymers, and such multicomponent binder systems can be conveniently used to further vary the physical properties of the admixture, e.g., to enhance adhesion.

As stated earlier, the term pigment refers herein to the total of both the photocond-uctive pigment and the non-photoccnductive insulating coloring agent or whitener. Other coloring agents and fillers of an insulating nature, relative to the photoconductive pigment employed, may also be used, particularly if some other sheet color is desired, but whitening agents and especially titanium dioxide are usually preferred because of the greater commercial importance of copysheets having the over-all appearance of a good white bond paper.

Although the techni-ques employed in coating these pigmented compositions onto the electrically conductive backings, such as aluminum foil and plastic which is vapor coated with aluminum, are essentially the same as those employed with the photoconductive compositions heretofore utilized (e.g., slurry of the photoconductive pigment in a solution of the polymeric binders in ethyl acetate, methyl ethyl ketone, acetone toluene or benzene), the dry coating weight and thickness generally is less, usually ranging from about l gram per square foot to about 5 grams per square foot preferably from 2 to 3 grams per square foot, permitting considerable economy in materials cost.

FIGURE 1 is a plot of percent reflection from image areas (pure white having percent reflection) versus exposure for various copysheets employing from 0 to 75 weight percent styrene homopolymer (Dow PS-Z,

"Styron, 20,000-35,000 molecular weight, Dow Chemical CO.) in the resinous binder. The copysheets were prepared by coating aluminum foil (2.5-3.0 grrr/ft2 dry weight) with a slurry in ethyl acetate of a composition having a 5:1 pigment:binder ratio. The pigment contained zinc oxide and 15 mol percent Ti02. A ten second exposure was made through a standard gray scale transparency With optical density units varying from 0.3 to 1.7. Electrolytic development was made using a constant plating time and 60 volts 11C. (no load) by passing a sponge wetted with a dilute aqueous solution of a 1:3 mol ratio silver-thiourea complex (connected as anode) over the exposed area, the aluminum foil backing being connected as cathode. As can be observed in FIG- URE 1, the binder compositions containing up to about 70 weight percent styrene homopolymer displayed both significantly greater contrast and speed than those cornpositions containing no styrene homopolymer and those compositions containing above about 70 weight percent styrene homopolymer.

FIGURE 2 represents a plot of percent reflection versus exposure for various copysheets. The same wet technique described above was used. Contrast is measured by the difference between the percent reilection in the image areas compared -to the original percent reflection of the sheet. The data shows the decreased response rate and decreased contrast obtained (with and without 15 Weight percent of TiO2 pigment) when the binder composition contained only Pliolite S-7 butadiene-styrene copolymer. The data further shows the improved response characteristics and sheet whiteness obtained when the resinous binder also included 50 weight percent of styrene homopolymer (Dow 13S-2), even when the composition contained weight percent of titanium dioxide based on total pigment. As the amount of polystyrene in the binder approached 100 percent, the contrast dropped markedly and the developed sheet was black even in the background areas under almost all exposure conditions.

A further series of illustrative runs will graphically show the improvement in response rate and contrast as related to the weight percent of styrene homopolymer in the binder system. No whitening `agent was employed for this series of runs, although similar results are obtained with a given amount of whitener incorporated therein. One square inch sample specimens were provided by coating clean aluminum foil with a 0.7 mil dry coating thickness of a zinc oxide/binder mixture (5.5/1 wt. ratio) in suitable solvent. After the coating was dry, the electrically conductive surface of a conductive glass plate (NESA glass, tin oxide coating) was placed in contact with the dry coating surface. The NESA glass electrode was connected as anode and the aluminum foil was connected as cathode, with a 30 volt D.C. potential applied across the electrodes. After a period of dark adaptation a 15 second exposure was made with 10 footcandles of incident light from a tungsten source. Current measurements in this dry test were taken at the end of the exposure, using a meter in the circuit. FIGURE 3 is a plot of the light conductance at varying weight percent values of styrene homopolymer (Dow 700) in the binder. The copolymer of butadiene and styrene was Pliolite S-7. It was also noted that the increase in light conductance was greater than the increase in dark conductance up to a value of about 70 weight percent of the polystrene.

The data set forth in FIGURE 5 illustrates the results achieved in the instant invention, using a wet test technique. At the higher values of polystyrene content in the binder, i.e., over 30 wt. percent, the measured conductance values difer somewhat between the wet and dry test methods, perhaps due to the eifect of electrolyte in the wet test. Though the quantitative extent of the measured improvementsy obtained is dependent on the particular test technique selected, both techniques can be used for qualitative determination. Wet test measurements are generally preferred when such copysheets are to be used in the electrolytic development method discussed earlier. The wet test technique utilized a one square inch test specimen having a 0.7 mil dry thickness coating of zinc oxide-binder mixture (5/1 Weight ratio) on a clean aluminum foil backing. The specimens were exposed for live seconds to 10 footcandles of incident light from a 150 watt tungsten source. With the aluminum foil connected as cathode, the exposed specimen was immersed in a developer bath (1:3 mol ratio silver nitrate-ethylene thiourea in dilute aqueous solution, 0.65 weight percent silver nitrate, 0.75 weight percent acetic acid, 5 weight percent magnesium acetate) and developed for 5 seconds using a 1.5 volt D.C. potential. An anode was immersed in the developer bath. Current flow was measured by a meter in the circuit, and conductance measurements were then calculated. In FIGURE 5 the rapid increase in the dark conductance above about 70 weight percent of vinyl benzene homo-polymer is shown. In this plot the vinyl benzene homopolymer was polystyrene (20,000-35,000 molecular weight) and the other polymer was butadiene-styrene copolymer (30:70 mol ratio respectively). Conductance values can, of course, be converted to conductivity values in conventional manner.

The earlier discussed d-ata appearing in FIGURE 3 illustrate the improvement in light conductance obtained with about 0.1 to about 70 Weight percent of vinyl benzene homoplymer in admixture with 70:30 mol ratio vinyl-benzene-butadiene copolymer. However, as stated earlier, such improvement is related to the mol percent ot vinyl benzene in the overall binder system, and experimental data has indicated that the light conductance of a vinyl benzene homopolymer-vinyl benzene copolymer binder system is generally comparable to a system having a single vinyl benzene-butadiene copolymer with an equivalent mol percent of vinyl benzene in the copolymer. Moreover, a binder system having vinyl benzene within the abovementioned molar concentrations may be prepared by admixing polybutadiene and polystyrene, although some diiiculty is encountered in the coating techniques when the higher pigment to binder ratios are used.

FIGURE 4 is a plot of the response rate (mbo/sec.) at varying weight percent values of styrene homopolymer in the resinous binder, using the same dry test techniques and polymers as described with respect to FIGURE 3. The increased response characteristics, particularly between about 5 and about 70 weight percent of homopolymer in the binder further indicates the greatly improved sensitivity of these copysheets. However, this response rate decreases somewhat with the incorporation of pigments such as TiO2, though the response rate remains comparatively high.

The use of sensitizing dyes in the photosensitive films or coatings of this invention broadens the spectral response of the photoconductor, and small quantities of those dyes, e.g., Phosphine R (Cl 788), Patent Blue (Cl. 672), xylene cyanol (Cl. 715), etc. increase the rate of response and the ratio of light to dark conductors.

The increased response rates obtained when the binders of this invention include small amounts of polystyrene tetramer illustrate the advantages of using more than one vinyl benzene homopolymer. Thus, when 1 weight percent of tetramer was incorporated into a binder containing azhljl weight ratio of Pliolite S-7 and Dow 700 homopolymer, the response rates calculated at 1/ 10, 2/ 10 and 3/ 10 seconds exposure under the dry test conditions described earlier (10 footcandles incident light from a tungsten source) were 200, 293 and 387 mho/sec. respectively. Without the tetramer inclusion the response rates at 1/10, 2/10 and 3/10 seconds were 73.3, and 120 respectively. This represents an approximate threefold increase in the rate of response and hence almost a threefold increase in light conductance for a given exposure time.

Additional experimental runs were made to confirm the improvement obtained with styrene homopolymers of various molecular weights. Zinc oxide (U.S.P. 12) was mixed with the binders of Table I in a 6:1 weight ratio. Each grind was ball milled in toluene (38.8 gm.) for 22 hours. To each grind was added 0.075 wt. percent of Eosin, Seto Flavin T and Solantin yellow dyes for sensitization, after which the grinds were further bail milled for 15 minutes. The grinds were then coated onto an aluminum vapor coated Mylar polyester lm at a Wet coating thickness of 1.5 mils. The resulting dry test data appears in Table` I.

As discussed earlier, the homopolymer employed may be of relatively low molecular weight, in which case smaller quantities of homopolymer are generally used to avoid problems of adhesion and tack. Table II illustrates the improved results obtained by adding polystyrene tetramer to a butadiene-styrene copolymer (30:70 mol ratio). A 5.5:1 zinc oxide to binder ratio was employed, and the slurry in toluene was ball milled for 4 hours. Sensitizing dyes (0.05 Weight percent Seto Flavin T and 0.05 weight percent Eosin) were added to the mix prior to coating onto the aluminum vapor coated plastic sheet. The light conductance was measured after a 15 second exposure to 10 footcandles of illumination from a watt tungsten source, using the dry test method r described earlier.

Table I Response Rate, mbo/sec., alter- Run Binder 1 Binder 2 1/10 sec 2/10 sec 3/10 sec.

Pliolite S-7 (28 gm.) 13.3X107- 23.3Xl0-7- 27.8X10-7. Pliolite S-7 (14 gm.) Dow 700 (14 gm.) 66.7X10-1 100 10"7 153 107 Pliolite S-7 (14 gm Dow PS-2 (14 gm 93.3X101 140 107 182 10-1 Pliolite S-7 (14 gm.) Dow PS-3 (14 gm). 10 107 167 107 220x10-7 Table II Response Rate, mbo/sec., alter- Run Wt. percent Wt. percent Light tctramer copolymer Conductance 0.5 99. 500 107 1,183X10-7. 1,727X10-7. 4,500X10"7. 1. 0 99. 0 5O0X10-7 1,200X10-7- 1,700X1O-7- 4,l50 107 2.0 98. 0 533)(10-7 1,220X10-7- 4 233)(10-7 5. 0 95. D 7O0X107 1,383Xl07. 1,867X107 4,233X107. 0. 0 100.0 500)(10-7 1,033X10-7- 1,467X10-1. 4,167X10-7.

The following typical formulations in Table III illustrate the comparative effect on copysheet whiteness of titanium dioxide and barium sulfate. It is noted from Table III that the titanium dioxide provides better reection than the barium sulfate. The dry test technique was employed.

Similar results are obtained with cadmium suliide, CdSe, CdTe, and indium oxide as the photoconductive material. The data in Table IV illustrates the improvement in photoconductivity achieved with cadmium sultide, using a 4:1 pigment to binder weight ratio. The vpigment-binder compositions Were ball milled for 24 hours in a toluene slurry and coated onto aluminum foil at a Wet thickness of 4 mils. The vinyl benzene homopolymer was Dow 700 and the butadiene copolymer Was Pliolite S-7. The increase in the photoconductivity when the binder mixture contains up to about 70 weight percent of the butadiene-styrene copolymer, particularly between about 30 and about 70 Weight percent, is evident from this data obtained with the dry test technique. Wet test techniques may also be used, With results similar to those obtained with zinc oxide.

The electrolytic image reproduction process, in Which the copysheets of this invention are used, has made possible the direct copying of microfilm reproductions of printed pages of books, papers and the like, with the same dimensions as the original and within a period of time of not more than about live or ten seconds from initial inspection of the light image to delivery of the completed print. The improved photoconductive copysheets described herein provide prints of greater contrast which have whiter highlight areas, as Well as the other adv-antages described earlier.

From the foregoing data and discussion, it becomes apparent that improved photoconductive copysheets for electrolytic image reproduction can be produced by utilizing an organic resinous binder which contains a vinyl benzene in polymerized form within a certain range of concentration, the remaining polymeric constituent being butadiene. As can be determined from the available data, the binder composition must contain from about 72 mol percent to .about 88 mol percent of a vinyl benzene in polymer form fand, correspondingly, from about 12 mol percent to about 28 mol percent of butadiene. In a preferred embodiment, this is achieved by admixing appropriate amounts of a vinyl benzene homopolymer with a butadiene-vinyl benzene copolymer, as mentioned earlier.

It will be understood by those skilled in the art that certain other embodiments as well las certain modifications of the illustrated embodiments are possible and are within the scope of this invention.

We claim:

1. A photoconductive copysheet capable of electrolytic development having :a thin, uniform layer of pigment particles comprising photoconductive particles and up to about 30 weight percent, based on total pigment particles, of 1an insulating, White, non-photoconductive inorganic pigment dispersed in a binder, said layer being -iirmly held to a continuous electrically conductive support, said binder comprising a substantially water-insoluble, non-tacky insulative organic resinous binder consisting essentially of from about 0.1 to labout 70 weight percent of a monovinyl benzene homopolymer Iand between about 30 and about 99.9 weight percent of a butadiene-vinyl benzene copolymer having from about 20 to about 50 mol percent but-adiene, the amount of said photoconductive particles being such that an electrically conductive path is created between the exposed surface of said layer and said electrically conductive support upon irradiation of said layer.

2. The photoconductive copysheet of claim 1 wherein said uniform layer contains between about and about weight percent of titanium dioxide, based on the total weight of pigment in said layer.

3. The photoconductive copysheet of claim 1 wherein said organic resinous binder contains from about 3() to about 60 weight percent of said butadiene-styrene copolymer, based on total weight of binder.

4. A photoconductive copysheet capable of electrolytic development having a thin, uniform layer of pigment particles comprising photoconductive particles land up to about weight percent, based on total pigment particles, of an insulating, white non-photoconductive inorganic pigment dispersed in a binder, s-aid layer being firmly held to a continuous electrically conductive support, said binder comprising a substantially water-insoluble, nontacky insulative organic resinous binder consisting essentially of from about 1 to about 70 weight percent of a high molecular weight monovinyl benzene homopolymer and between about 30 and about 99 weight percent of a butadiene-vinyl benzene copolymer having from about 2() to about 50 mol percent butadiene, the amount of said photoconductive particles being such that an electrically conductive path is created between the exposed surface of said layer land said electrically conductive support upon irradiation of said layer.

5. The photoconductive copysheet of claim 4 wherein said white non-photoconductive pigment is titanium dioxide.

6. A photoconductive copysheet capable of electrolytic development having a thin, uniform layer of pigment particles comprising photoconductive particles and up to about 30 weight percent, based on total pigment particles, of an insulating, white, non-photoconductive inorganic pigment dispersed in a binder, said layer being rmly held to a continuous electrically conductive support, said binder comprising a substantially water-insoluble, non-tacky insulative organic resinous binder consisting essentially of from about 1 to about 70 weight percent of a low molecular weight monovinyl benzene homopolymer and between about 30 and about 99 weight percent of a butadiene-vinyl benzene copolymer having from Iabout 2O to about mol percent butadiene, the amount of said photoconductive particles being such that an electrically conductive path is created between the exposed surface of said layer and said electrically conductive support upon irradiation of said layer.

7. The photoconductive copysheet of claim 6 wherein said white, non-photoconductive pigment is titanium dioxide.

8. ln a photoconductive copysheet suitable for use in the electrolytic image reproduction process and having a thin, uniform, continuous layer of photoconductive particles in a binder adhered to a smooth, continuous electrically conductive surface, the improvement wherein said binder comprises a substantially water-insoluble, non-tacky, insulative organic resin mixture having between about 0.1 and weight percent of a monvinyl benzene homopolymer and between about 30 and 99.9 weight percent of a butadiene-vinyl benzene copolymer having from about 20 to about 50 mol percent butadiene, said continuous layer also having between about l0 and about 30 weight percent of an insulating, white, non-photoconductive inorganic pigment.

9. The copysheet of claim 1 in which said photoconductive particles are particles of zinc oxide.

10. The copysheet of claim 1 in which said photoconductive particles are particles of cadmium sulfide.

1l. The copysheet of claim 1 in which said photoconductive particles are particles of indium oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,574,439 11/51 Seymour 260-45.5 2,727,878 12/55 Ballman 26045.5 2,955,938 10/ 60 Steinhilper 96-1 2,990,279 6/61 Crumley 96-1 3,010,884 11/61 Johnson 96-1 3,041,168 6/62 Wielicki 96--1 NORMAN G. TORCHIN, Primary Examiner. 

1. A PHOTOCONDUCTIVE COPYSHEET CAPABLE OF ELECTROLYTIC DEVELOPMENT HAVIG A THIN, UNIFORM LAYER OF PIGMENT PARTICLES COMPRISING PHOTOCONDUCTIVE PARTICLES AND UP TO ABOUT 30 WEIGHT PERCENT, BASED ON TOTAL PIGMENT PARTICLES, OF AN INSULATING, WHITE, NON-PHOTOCONDUCTIVE INORGANIC PIGMENT DISPERSED IN A BINDER, SAID LAYER BEING FIRMLY HELD TO A CONTINUOUS ELECTRICALLY CONDUCTIVE SUPPORT, SAID BINDER COMPRISING A SUBSTANTIALLY WATER-INSOLUBLE, NON-TACKY INSULATIVE ORGANIC RESINOUS BINDER CONSISTING ESSENTAILLY OF FROM ABOUT 0.1 TO ABOUT 70 WEIGHT PERCENT OF A MONOVINYL BENZENE HOMOPOLYMER AND BETWEEN ABOUT 30 AND ABOUT 99.9 WEIGHT PERCENT OF A BUTADIENE-VINYL BENZENE COPOLYMER HAVING FROM ABOUT 20 TO ABOUT 50 MOL PERCENT BUTADIEN, THE AMOUNT OF ASIA DPHOTOCONDUCTIVE PARTICLES BEING SUCH THAT AN ELECTRICALLY CONDUCTIVE PARTICLES BEING SUCH THAT AN ELECTRICALLY CONDUCTIVE PATH IS CREATED BETWEEN THE EXPOSED SURFACE OF SAID LAYER AND SAID ELECTRICALLY CONDUCTIVE SUPPORT UPON IRRADIATION OF SAID LAYER. 